1. Field of the Invention
The present invention relates to speech encoding and decoding, and more particularly, to a high-band speech encoding apparatus and a high-band speech decoding apparatus in wideband speech encoding and decoding with a bandwidth extension function, and a high-band speech encoding and decoding methods performed by the apparatuses.
2. Description of Related Art
As the field of application of speech communications broadens, and the transmission speed of networks improves, and the necessity for high-quality speech communications becomes more imminent. The transmission of a wide-band speech signal having a frequency range of 0.3 to 7 kHz, which is excellent in various aspects such as naturalness and clearness compared to an existing speech communication frequency range of 0.3 to 3.4 kHz, will be required.
On a network side, a packet switching network which transmits data on a packet-by-packet basis may cause congestion in a channel, and consequently, damage to packets and degradation of the quality of sound may occur. To solve these problems, a technique of hiding a damaged packet is used, but this is not a fundamental solution.
Accordingly, a wideband speech encoding/decoding technique that can effectively compress the wideband speech signal and also solve the congestion of a channel has been proposed.
Currently-proposed wideband speech encoding/decoding techniques may be classified into a technique of encoding a complete speech signal having a frequency range of 0.3 to 7 kHz all at a time and decoding the encoded speech signal and a technique of hierarchically encoding frequency ranges of 0.3 to 4 kHz and 4 to 7 kHz into which the speech signal having the frequency range of 0.3 to 7 kHz is divided, and decoding the encoded speech signal. The latter technique is a wideband speech encoding and decoding technique using a bandwidth extension function that achieves optimal communication under a given channel environment by adjusting the amount of data transmitted by layers according to a degree of congestion of a channel.
In the wideband speech encoding using the bandwidth extension function, a high-band speech signal having a frequency range of 4 to 7 kHz is encoded using a modulated lapped transform (MLT) technique. A high-band speech encoding apparatus employing the MLT technique is the same as a high-band speech encoding apparatus 100 shown in FIG. 1.
Referring to FIG. 1, the high-band speech encoding apparatus 100 includes an MLT unit 101 that receives a high-band speech signal and performs MLT on the high-band speech signal to extract an MLT coefficient. The amplitude of the MLT coefficient is output to a 2 dimension-discrete cosine transform (2D-DCT) module 102, and a sign of the MLT coefficient is output to a sign quantizer 103.
The 2D-DCT module 102 extracts 2D-DCT coefficients from the amplitude of the received MLT coefficient and outputs the 2D-DCT coefficients to a DCT coefficient quantizer 104. The DCT coefficient quantizer 104 orders the 2D-DCT coefficients from a 2D-DCT coefficient with a largest amplitude to a 2D-DCT coefficient with a smallest amplitude, quantizes the ordered 2D-DCT coefficients, and outputs a codebook index for the quantized 2D-DCT coefficients. The sign quantizer 103 quantizes a sign of the MLT coefficient having the largest amplitude.
The codebook index and the quantized sign are transmitted to a high-band speech decoding apparatus 110, which decodes the encoded high-band speech signal through a process performed in the opposite order to the process of the high-band speech encoding apparatus 100 and outputs a decoded high-band speech signal.
However, when a speech signal is transmitted at a low bitrate, the high-band speech signal encoding based on the MLT technique cannot guarantee restoration of high-quality sound. As the bitrate decreases, the degradation of sound restoration performance becomes prominent.